A Passive Optical Network, PON, is a point-to-multipoint network architecture employing fibre cables from a central office to premises. It employs unpowered optical splitters to enable a single optical fibre to serve multiple premises. A PON comprises an Optical Line Terminal, OLT, at the central office of the service provider. It comprises a number of Optical Network Terminals, ONTs, near end users. A PON configuration reduces the amount of fibre and central office equipment required compared with point-to-point architectures. A passive optical network is a form of fibre-optic access network.
In order to supervise and monitor the performance of a PON, Optical Time-Domain Reflectometry, OTDR, is used. An OTDR device injects a series of optical pulses into the fibre. The series of optical pulses, also called OTDR signal travel down the network towards the ONTs. Parts of the OTDR signals are reflected back towards the OTDR device. The back-reflected, or back-scattered, OTDR signal may be used for estimating the fibre's length and overall attenuation, including splitter losses. The back-scattered OTDR signal may also be used to locate faults, such as breaks, and to measure optical return loss.
Generally, some requirements are placed on a monitoring or supervision system. Monitoring should not influence regular data communication, i.e. it should be non-invasive. This is achievable by utilisation of a dedicated optical bandwidth for the measuring function. Further, the technique should be sensitive to relatively low power fluctuations detectable in on-demand or periodic modes. Still further, it should not require any high initial investment. This mainly yields that no additional monitoring functionality on the ONT side should be needed and PON monitoring functionality should be shared over a complete PON system or a group of PON systems.
The today's existing solutions for providing supervision or monitoring do only satisfy some of the above requirements. Most of the solutions existing today significantly increase capital expenditures because they require either a customised OTDR device, which is expensive, wavelength specific components in the fibre links (drop links) towards the ONTs, which causes power budget reduction, advanced OLT transmitter upgrades, e.g. light path doubling. Still further, most of today's existing solutions to provide supervision or monitoring can only detect a fault in a fibre link which introduces significant loss of more than 5 dB, far above an expected threshold of 1 dB.